The described apparatus is an improved cruise speed control system for commercial jet aircraft, and more specifically, is the circuit or system for using aircraft pitch as well as throttle control for maintaining the aircraft at the commanded speed.
In light of recent increases in fuel costs, airline operational economics have dictated that airplanes be flown at, or close to, their maximum specific air range cruise speeds for best fuel economy. Unfortunately, in the vicinity of this cruise speed, today's low-drag airplines have virtually neutral speed stability; i.e. aircraft speed is very sensitive to small changes in engine thrust. It is possible to manually control the airplane speed to this optimum fuel-efficient Mach number (M), but it requires continuous attention by the flight crew to the task of throttle management. Pilots generally cannot consistently respond to this high required work load, and as a result, Mach number can vary radically from the optimum. More often, the captain simply elects to fly at a higher Mach, where the airplane is strongly speed stable but much less fuel-efficient.
More specifically, airplane and engine performance trends at typical cruise altitudes and weights show that the change in engine thrust is very small over a wide Mach number, i.e., the speed stability on thrust is essentially zero. The result is that small changes in engine thrust produce large Mach number changes, and conversely, large Mach number changes can occur at constant throttle settings.
Simple advisory systems containing engine thrust maps and airplane performance data are already on the market. These systems merely inform the pilot of the optimum flight parameters for any given flight condition. However, the pilot is still required to control speed manually (or use over-active autothrottles) and the basic problem of pilot workload, annoying thrust changes, and the associated ramifications remain.
Simple cruise autothrottle systems can be used to stabilize Mach number somewhat, but such operation is generally characterized by excessive throttle activity which is annoying to the flight crew and passengers, and may have adverse effects on engine life and fuel consumption.
The basic problem, then, which is common to all of today's modern jet aircraft and in particular jet transports, is that efficient operation requires an automatic system for more effectively maintaining aircraft speed.